1. Field of the Invention
The present invention relates to a circuit for improving the distortion performance of amplifiers, and more particularly, to a linearization circuit to improve the distortion performance of broadband hybrid RF amplifiers.
2. Description of Related Art
In transferring and distributing signals, most cable television (CATV) networks employ hybrid fiber/coax (HFC) networks. A typical HFC network includes optical transmitter and nodes, and distribution amplifiers to provide optical and electrical paths for the signals, respectively. (An optical node converts between the optical and RF domains.) Effective transfer of the signals along the optical and electrical paths requires output stage signal amplification at the optical transmitter and nodes, and the distribution amplifiers. For this amplification, hybrid RF amplifiers, which typically employ push-pull and power-doubling techniques to improve their distortion performance, are commonly used. Such hybrid RF amplifiers are commercially available from vendors such as Philips and Motorola.
When the coaxial cable portion of an HFC network is passive, that is, there is no distribution amplifier in the network, the distortion from the node output stage amplifier may dominate the total system distortion performance. At such a passive coaxial cable portion, it is desirable to drive the output stage amplifier in the node at maximum power level in order to transmit the signal further. However, higher output power can produce worse distortion performance. Therefore, the optical nodes need an enhanced output stage amplifier to improve the distortion performance at high output power.
There are several known linearization techniques that improve distortion performance of output stage amplifiers. Among them are predistortion and feedforward circuits which are schematically shown in FIGS. 1A and 1B, respectively. Even though both techniques can improve the distortion performance, they have their own limitations.
Referring to FIG. 1A, the feedforward circuit includes an error amplifier 20 to cancel the distortion produced by an output amplifier 10. A first directional coupler 1 splits an input signal onto two paths 12 and 14. On path 12, output amplifier 10 amplifies the signal, and then a second directional coupler 2 splits the amplified signal onto paths 12 and 16. On path 16, an attenuator 3 attenuates the signal, and a phase-shifter 4 phase-shifts the signal by 180.degree.. At a third directional coupler 7, the signal from path 16 and the signal from path 14 through a first delay line 5 combine together. As a result, fundamental portions of the signals from paths 14 and 16 cancel each other, and distortion portions of the signals are coupled at the output of third directional coupler 7. Then, error amplifier 20 amplifies the combined signal which is a distortion signal.
The signal from second directional coupler 2 on path 12 passes through a second delay line 6, and is combined with the amplified signal from error amplifier 20 at a third directional coupler 30. When the output signals combine together at a combiner 30, the distortions in both output are equal in magnitude but opposite in phase, so that the distortions cancel each other. This feedforward technique is known for its high distortion cancellation capability. For example, this feedforward technique can cancel all orders of distortions whereas other linearization techniques only cancel one type of distortion. Thus, the feedforward technique is widely used in broadband applications where reactive components have different effects at different frequencies. However, the feedforward technique has several shortcomings. For example, multiple output amplifiers cannot be coupled to a single feedforward circuit, and there can be signal loss at the output stage. Driving the output amplifier at a higher level to overcome the loss at the output stage may introduce more distortions.
The predistortion circuit shown in FIG. 1B does not introduce any loss at the output stage of output amplifier 10. This circuit employs a distortion generator 40 to produce predistortion from a signal that was split from an input signal through a first directional coupler 22, and provides the predistortion into the input stage of error amplifier 20. At a second directional coupler 24, the other signal that was split from an input signal through a first directional coupler 22 passes through a delay line 25 combines with the signal from error amplifier 20. Then, the combined signal is inputted into output amplifier 10.
The predistortion from distortion generator 40 through error amplifier 20 cancels the distortion from amplifier 10 at the output stage of output amplifier 10. Typically, distortion generator 40 does not have similar distortion characteristics as does amplifier 10, limiting the circuit's cancellation capability.
Diode-based distortion generators, which are widely used for predistortion, have disadvantages when employed in hybrid RF amplifier linearization. First, different orders of distortions may require separate, different distortion generators. That is, separate distortion generators are required to cancel second and third order distortions. This increases cost and circuit complexity. Second, when the hybrid RF amplifiers are driven at high output power, higher (fourth and fifth) order distortions become more pronounced, and overlap on lower order distortion frequencies. Such diode distortion generators cannot effectively cancel the high order distortions.
U.S. Pat. No. 5,258,722, incorporated herein by reference in its entirety, discloses a distortion cancellation circuit including a distortion generating circuit 50, as shown in present FIG. 2. Distortion generator 50 includes a first intermediate amplifier 36, which is ideally identical to each of four output amplifiers 60-1 to 60-4, and a second intermediate amplifier 42. In distortion generator 50, a first splitter 53 splits an input signal onto two paths 32 and 34. The signal on path 32 passes through amplifier 36, a first attenuator 54, and a phase-shifter 57, and the signal on path 34 passes through amplifier 42 and a second attenuator 55. The signals on both paths 32 and 34 combine together at a combiner 56, and the combined signal is inputted into output amplifiers 60-1 to 60-4 through a second splitter 58.
Amplifiers 36 and 60-1 to 60-4 are intended to be driven at the same output power. Theoretically, distortion generating circuit 50 may generate a predistortion that can cancel the distortion generated from output amplifiers 60-1 to 60-4. However, distortion generating circuit 50 may have several shortcomings in practice.
For effective distortion cancellation, output amplifiers 60-1 to 60-4 must be of high gain (30 dB or more) in order to have amplifier 42 driven at far lower power level than are output amplifiers 60-1 to 60-4, so as to compensate for the excessive signal loss in distortion generating circuit 50. Otherwise, amplifier 42 must be driven at much higher power level. The distortion generated from amplifier 42 will become dominant, and the magnitude of the predistortion generated at combining point 56 will not be proper to cancel the distortion generated from output amplifiers 60-1 to 60-4.
Commercially available hybrid RF amplifiers used for forward transmission typically have gain of 18 to 21 dB. If such amplifiers are used for amplifier 36, amplifier 42 will be driven at only a few dB below the output power level of output amplifiers 60-1 to 60-4. In addition, higher gain amplifiers are very difficult to make, and not commercially available.
Another shortcoming is that the fundamental signals from paths 32 and 34 must be subtracted from each other at combining point 56 to obtain the out-of-phase distortion component. The fundamental signal power difference at combining point 56 is 6 dB. When the signal power difference is small as in this case, a slight imbalance in phase and magnitude of signals will degrade frequency response flatness which may affect the optical node frequency response flatness requirement. In addition, the poor flatness will cause the distortion generated by output amplifiers 60-1 to 60-4 to be less similar to the distortion generated by amplifier 36, and in turn will result in incomplete distortion cancellation.